The Janus kinase (JAK) family is one of the well recognized families of non-receptor tyrosine kinases. The JAK family may be activated by the binding of a cytokine to a cell surface receptor. Activated Jak may then initiate intracellular signaling cascades. The Jak family and Signal Transducers and Activators of Transcription (STATs) are involved in the signaling pathways of a wide range of cytokines.
The JAK/STAT pathway has been shown to play a role in inflammatory diseases, such as, inflammatory diseases of the respiratory tract, multiple sclerosis, rheumatoid arthritis, asthma, inflammatory bowel disease, allergies, autoimmune diseases and other immune reactions. The JAK/STAT pathway, for example, JAK3/STAT, may also play a role in cancers.
Inhibitors of the Jak family are widely sought after and published for treatment or prevention of inflammatory diseases or cancers.
The Janus kinase family of protein tyrosine kinases (JAKs) may play a role in the cytokine-dependent regulation of proliferation and function of cells involved in immune response. Four mammalian JAK family members have been reported: JAK1 (also known as Janus kinase-1), JAK2 (also known as Janus kinase-2), JAK3 (also known as Janus kinase, leukocyte; JAKL; L-JAK and Janus kinase-3) and TYK2 (also known as protein-tyrosine kinase 2). The JAK proteins may range in size from 120 to 140 kDa and comprise seven conserved JAK homology (JH) domains; one of those can be a functional catalytic kinase domain, and another can be a pseudokinase domain potentially serving a regulatory function and/or serving as a docking site for STATs (Scott, Godshall et al. Clin. Diagniabimmunol, 9 (6): 1153-1159, 2002). While JAK1, JAK2 and TYK2 can be ubiquitously expressed, JAK3 is reported to be expressed, for example, in natural killer (NK) cells and not resting T cells, suggesting a role in lymphoid activation (Kawamura, M., D. W. McVicar, et al. “Molecular cloning of L-JAK, a Janus family protein-tyrosine kinase expressed in natural killer cells and activated leukocytes.” Proc Natl Acad Sci USA 91(14): 63748, 1994).
The JAK/STAT pathway reportedly plays a role in the pathogenesis of the asthmatic response, chronic obstructive pulmonary disease, bronchitis, and other related inflammatory diseases of the lower respiratory tract (“JAK-STAT signaling in asthma.” J Clin Invest 109(10): 1279-83, 2002 The JAK/STAT pathway has also been implicated to play a role in inflammatory diseases/conditions of the eye including, but not limited to, iritis, uveitis, scleritis, conjunctivitis, as well as chronic allergic responses. Therefore, inhibition of JAK kinases may have a beneficial role in the therapeutic treatment of those diseases.
Inhibition of the JAK kinases can be also envisioned to have therapeutic benefits in patients suffering from skin immune disorders such as psoriasis, and skin sensitization. In psoriasis vulgaris, the most common form of psoriasis, it may have been generally accepted that activated T lymphocytes can be important for the maintenance of the disease and its associated psoriatic plaques (Gottlieb, A. B., et al, Nat Rev Drug Disc., 4:19-34, 2005). Psoriatic plaques may contain a significant immune infiltrate, including leukocytes and monocytes, as well as multiple epidermal layers with increased keratinocyte proliferation. While the initial activation of immune cells in psoriasis may occur by an ill defined mechanism, the maintenance has been reported to be dependent on a number of inflammatory cytokines, in addition to various chemokines and growth factors (JCI, 113:1664-1675, 2004). Many of those, including interleukins −2, −4, −6, −7, −12, −15, −18, and −23 as well as GM-CSF and IFNg, may signal through the Janus (JAK) kinases (Adv Pharmacol. 47: 113-74, 2000). As such, blocking signal transduction at the level of JAK kinases may result in therapeutic benefits in patients suffering from psoriasis or other immune disorders of the skin.
Blocking signal transduction at the level of the JAK kinases may also hold promise for developing treatments for human cancers. Cytokines of the interleukin 6 (IL-6) family, which may activate the signal transducer gp130, may be major survival and growth factors for human multiple myeloma (MM) cells. The signal transduction of gp130 is believed to involve JAK1, JAK2 and TYK2 and the downstream effectors STAT3 and the mitogen-activated protein kinase (MAPK) pathways. In IL-6-dependent MM cell lines treated with the JAK2 inhibitor tyrphostin AG490, JAK2 kinase activity and ERK2 and STAT3 phosphorylation may be inhibited. Furthermore, cell proliferation may be suppressed and apoptosis may be induced (De Vos, J., M. Jourdan, et al. “JAK2 tyrosine kinase inhibitor tyrphostin AG490 downregulates the mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription (STAT) pathways and induces apoptosis in myeloma cells.” Br J Haematol 109(4): 823-8, 2000). However, in some cases, AG490 can induce dormancy of tumor cells and can then protect them from death.
It has been suggested that inhibition of JAK2 tyrosine kinase can be beneficial for patients with myeloproliferative disorder. (Levitt, et al., Cancer Cell, vol. 7: 387-397, 2005). Myeloproliferative disorder (MPD) may include polycythemia vera (PV), essential thrombocythemia (ET), myeloid metaplasia with myelofibrosis (MMM), chronic myelogenous leukemia (CML), chronic myelomonocytic leukemia (CMML), hypereosinophilic syndrome (HES), and systemic mast cell disease (SMCD). Although the myeloproliferative disorders (such as PV, ET and MMM) are thought to be caused by acquired somatic mutation in hematopoietic progenitors, the genetic basis for these diseases has not been known. However, it has been reported that hematopoietic cells from a majority of patients with PV and a significant number of patients with ET and MMM possessed a recurrent somatic activating mutation in the JAK2 tyrosine kinase. It has also been reported that inhibition of the JAK2V617F kinase with a small molecule inhibitor led to inhibition of proliferation of hematopoietic cells, suggesting that the JAK2 tyrosine kinase can be a potential target for pharmacologic inhibition in patients with PV, ET and MMM.